Thermal actuators, such as wax motors, may be provided to convert temperature change into a force. Upon heating the thermal actuator, fluid disposed in the actuator is expanded, which causes a diaphragm to deform, thereby causing an axial movement of a piston adjacent the diaphragm. When the fluid is cooled, the fluid is contracted, which causes the diaphragm to return to its original state. Thermal actuators may be used in valves to open and close flow passages inside the valves. To minimize pressure drops caused by the thermal actuators, large diameter fluid tubes may be used.